1. Field of the Invention
The present invention relates to a method of manufacturing a micro-electrical-mechanical system.
2. Description of the Related Art
It is known art to use semiconductor fabrication techniques in micro-machining processes that create structures with sizes on the order of a few hundred micrometers. Applications of this technology are beginning to appear in sensors, optical communication switches, radio-frequency (RF) devices, and other areas.
Because these tiny structures can be manufactured by conventional semiconductor fabrication processes, they can be integrated together with signal processing circuitry, for example, onto a single chip. Chips of this type, which combine electronics with mechanics, are known as micro-electrical-mechanical systems (MEMS), and the technology they embody is known as microsystem technology (MST). The chips will be referred to as MEMS devices below.
Acceleration sensors are a well-known example of a MEMS device. A method of manufacturing a piezoelectric acceleration sensor is described in Japanese Unexamined Patent Application Publication No. H7-225540. First a plurality of masses and a plurality of beams movably supporting the masses are formed on a silicon substrate; then the masses and beams are reinforced with a polyimide film, and the substrate is diced into chips, each including one mass and its supporting beams, which are fabricated as a unitary structure. Finally, the polyimide film is removed from each chip, so that the mass can move in response to acceleration.
The need to apply a polyimide reinforcing film and then remove the film individually from each chip after dicing makes this manufacturing method undesirably lengthy and complex. A further problem is low yield, because despite the reinforcement, the beams are vulnerable to damage during the dicing process. The manufacturing cost per chip is accordingly comparatively high.